crypto: add _notmuch_crypto_decrypt wrapper function

We will use this centralized function to consolidate the awkward
behavior around different gmime versions.

It's only invoked from two places: mime-node.c's
node_decrypt_and_verify() and lib/index.cc's
_index_encrypted_mime_part().

However, those two places have some markedly distinct logic, so the
interface for this _notmuch_crypto_decrypt function is going to get a
little bit clunky.  It's worthwhile, though, for the sake of keeping
these #if directives reasonably well-contained.